The present disclosure relates to finned resistance spot welding electrodes.
Sheet steel has historically been the primary material used in automobile construction; however, significant quantities of specialized materials, including aluminum, TRIP steels, duplex steels, dual phase steels and coated high strength steels are currently also being used. The selection of these new materials is often based on either reducing the weight of the vehicle to meet improved fuel economy standards or increasing the strength of the vehicle to meet new safety regulations for crash situations. Although a variety of joining processes are utilized, resistance spot welding (RSW) is the primary process used to join members in assembly operations. This process is favored due to its inherently low capital and operating costs as well as its adaptability for use in mass production. However, high standards of weld quality and repeatable reliability are needed to take advantage of these other benefits.
For example, TRIP steels (transformation induced plasticity steels) include three separate phases of material. Traditional RSW techniques have proven to provide less than satisfactory results as traditional RSW techniques have resulted in low weld quality due to embrittled welds. It has been theorized that traditional RSW techniques result in a rapid heat up and cool down, wherein the rapid cool down, when used on TRIP steel, embrittles the weld nugget resulting in the reduction in strength of the weld.
In response to this phenomenon, manufacturers have altered their weld cycles to provide a gradual reduction in temperature by incrementally decreasing the current during cool down instead of simply turning the current off. While this appears to reduce the problem with embrittled welds, it places a higher heat load on the welding electrode as high heat conditions are maintained for a greater period of time. This can lead to premature electrode failures and quality control problems.
Another problem that has been encountered in RSW manufacturing environments is electrode wear. Electrode wear primarily occurs in two ways. First, electrode wear can result from extrusion of electrode material from the face of the electrode during welding operations. This phenomenon has been termed “mushrooming.” This may occur due to a combination of high heat generated during the welding process and high clamping force applied by the electrodes to the parts.
A second mechanism of electrode wear involves disposition of alloys from the electrode face onto the sheet steel surface during the retraction of electrodes. Heating the electrode face promotes brass alloying and bonding of the top layer of the electrode face in the seal surface. This both enlarges and roughens the electrode face.
These deposited alloys are not easily detected as they may appear to be similar to galvanized coatings. Traditional solutions to electrode wear include controlling the operating current level, weld time, hold time, electrode cooling, electrode geometry and other factors. The rate of electrode face enlargement from these two mechanisms is related to the energy generated at the electrode sheet interface and the heat passing from the weld to the electrode face to a water cooled channel within the electrode. Heat can reduce the material strength of the electrode by annealing the cold work structure of the electrode. These kinds of electrode degradation decrease the current density in the steel sheet which results in reduced heat at the weld site that may eventually cause a reduced nugget size or even the elimination of a weld altogether.
The effects of electrode wear can be alleviated by either systematically increasing the weld current to account for the greater size of the welding electrodes caused by “mushrooming” or by re-conditioning (dressing) the electrode tip and face to return the electrode surface to its original size.